Return to the boil, and whisk in the remaining 60 g of butter
and 25 g of finely diced beet
. Cut 150 g of onion
into 1 cm cubes, separating the layers. Place
in a medium saucepan with the 20 g butter
gently, covered. When almost cooked, cut up the zest of an untreated lemon
very finely and add to the pan.